Alignment - The mechanical condition of adjustable components within the vehicle's suspension. When a vehicle is in alignment, the caster, camber, toe-in and thrust settings are set to specification. Severe impacts (hitting potholes or curbs) and worn suspension parts are the leading causes of misalignment.

All Season Tires - Tires designed to provide good traction in a wide variety of road conditions, including wet, dry and mud and snow. This design also limits the tire’s performance in extreme conditions, or when compared to tires built for a particular category.

Alphanumeric - A term for describing the size of a tire (H78-15, for example) where both letters and numbers are used.

Aspect Ratio - A term that describes a tire’s height-to-width proportion. If a tire’s sidewall height were 65% of its section width, its aspect ratio would be 65. In the tire size expressed as 205/65-15, the number 65 is the aspect ratio.

Balance - The state in which a tire and wheel assembly spins with all its weight distributed equally. A wheel balancer is used to place weights compensating for static and dynamic imbalances that exist in all assemblies. Not balancing an assembly can result in vibration.

Bead - A round hoop of steel wires, wrapped or reinforced by steel cords, placed at the very inside of the tire's diameter. 

Bias Ply Tire - A pneumatic tire manufactured such that the plies are laid at alternate angles less than 90 degrees to the centerline of the tread. These criss-cross plies give the tire its strength, but generate heat during operation and limit the tire's wear and performance.

Carcass - The tire body beneath the tread and sidewalls; also called the casing.

Contact Patch - The portion of the tread that contacts the road during operation.

Cord - The strands of material forming the plies or layers of tire. Cords may be made from fiberglass, rayon, nylon, polyester or steel.

DOT Markings - Each tire has a required Department of Transportation number imprinted on at least one of its sidewalls. That number begins with the letters "DOT" and may contain up to 12 additional numbers and letters.

The first and last digits are the most important for the tire owner. The first two letters/numbers identify the manufacturer of the tires. Prior to the year 2000, the last three digits of a DOT number represented the week (two digits) and the year (one digit) of production.

For example, if the last three digits are 439, the tire was produced in the 43rd week of 1999. Tires produced after January 1, 2000 have a four digit date code at the end of the DOT number. The first two digits represent the week of production and the last two digits represent the last two digits of the year of production. So, 3500 as the last four numbers indicates that the tire was produced in the 35th week of the year 2000.

Footprint - The portion of the tire that makes contact with the surface of the road.

Friction - The resistance of one material (the tire tread) as it moves against another (the road); this is the force that causes the tire to grip to the road.

Gross Vehicle Weight - The actual weight of a vehicle when fully loaded with passengers and cargo.

Groove - The space between two adjacent tread ribs; also called tread grooves.

Heat Cycling - A method of "breaking in" competition tires prior to initial use. Heat cycling gradually heats the tire in a controlled environment to gently stretch the tread compound, resulting in better traction and longer tread life.

Highway Tires - Also called summer tires; designed for dry and occasional wet weather driving, but not for use on snow and ice.

Hydroplaning - A skimming effect caused by tires losing contact with a surface covered by water.

Innerliner - The innermost layer of a tubeless tire which prevents air from permeating through the tire. This thin layer of material replaces the innertube.

Load Index - An assigned number ranging from 0 to 279 that corresponds to the load carrying capacity of the tire.

M+S, M/S or M & S (Mud and Snow) - Indicates that a tire can reach particular standards for performance in mud and snow conditions. The tire must meet the Rubber Manufacturers Association (RMA) definition of a mud and snow tire.

Maximum Inflation Pressure - The maximum air pressure to which a cold tire may be inflated; found molded onto the tire's sidewall.

OE and OEM - OE means "Original Equipment" and refers to the tires included with a new vehicle at the time of purchase. The vehicle's manufacturer selects these tires to provide the optimal performance based on the performance characteristics of the vehicle. "OEM" stands for "Original Equipment Manufacturer."

Overall Diameter - The diameter of an inflated tire without any load.

Overall Width - The distance between a tire's outside sidewalls, including lettering and designs.

P Metric - Uniform designation of tire sizes in metric measurements originally introduced by American tire manufacturers in 1977. Commonly called "P-metric series." A typical P-metric tire size is P215/70R-15.

Placard - A small label typically located on the edge of the driver's door or inside the glove compartment of a vehicle. A placard contains information on the vehicle such as the manufacturer's recommended tire inflation pressure, seating capacity, and Gross Vehicle Weight (GVW).

Plus-Sizing - An option allowing drivers to customize the appearance and performance of their vehicle by mounting lower profile tires on larger diameter wheels. One-inch greater wheel diameter is referred to as plus-one, two inches is plus-two... and so on. Using a lower profile tire with a greater diameter rim allows the overall diameter to remain about the same.